The invention relates to a process for preparing N-alkyl-Nxe2x80x2-methylalkyleneureas and, in particular, to the preparation of N,Nxe2x80x2-dimethylpropylene- and -ethyleneurea, which are essentially free of formic acid.
N,Nxe2x80x2-Dimethylpropyleneurea (DMPU) and the analogous N,Nxe2x80x2-dimethylethyleneurea (DMEU) are important as special polar aprotic solvents in agrochemical and drug syntheses. Particularly in reactions with carbanions or carbanion equivalents, DMPU or DMEU can replace the carcinogenic hexamethylphosphoramide. In addition, these N,Nxe2x80x2-dimethylalkyleneureas are employed in chemical engineering, for example in extractive distillations to remove cyclohexene from a mixture with benzene and cyclohexane.
The so-called Leuckart-Wallach reaction in particular has become important for preparing such tetrasubstituted ureas and entails reaction of propyleneurea with aldehydes in the presence of formic acid, as described in EP-A 0 301 270.
EP-A 0 280 781 describes the preparation of N-alkyl-Nxe2x80x2-methylalkyleneureas and, in particular, of N,Nxe2x80x2-dimethylalkyleneureas such as DMPU from propyleneurea and formaldehyde in the presence of formic acid by the so-called Eschweiler-Clark variant.
However, a problem with this is the removal, which should be as complete as possible, by distillation of the formic acid necessary for this reaction. According to EP-A 0 280 781, for complete removal of the formic acid either bases such as alkali metal or alkaline earth metal hydroxides or carbonates are added, or a lower alcohol and a catalytic amount of a mineral acid which serves to esterify the remaining formic acid still present. The ester is then removed by distillation.
Another possibility for essentially complete removal of the formic acid after the reaction is described in EP-A 0 356 973. The formic acid still present in the reaction mixture after the reaction is subjected therein to a thermal decomposition reaction at temperatures of about 120 to 260xc2x0 C. using a catalyst system composed of a tertiary amine and a copper salt such as CuCl.
It is an object of the present invention, based on this prior art, to develop a process which makes it possible to prepare essentially formic acid-free N-alkyl-Nxe2x80x2-methylalkyleneureas and, in particular, to prepare DMPU from propyleneurea and formaldehyde under Eschweiler-Clark conditions without elaborate workup steps.
We have found that this object is achieved by a process for preparing N-alkyl-Nxe2x80x2-methylureas which have the formula represented below 
with R1=H or CH3, R2=CnH2nxe2x88x921 with n=1-4 and x=0 or 1, from the corresponding alkyleneureas by reaction with monomeric or polymerized formaldehyde in the presence of formic acid, in which an essentially formic acid-free final product is obtained by feeding the mixture, obtained in the reaction, of N-alkyl-Nxe2x80x2-methylalkyleneurea and formic acid to the upper region of a distillation column, distilling without further additions and removing essentially formic acid-free N-alkyl-Nxe2x80x2-methylalkyleneurea in the lower region of the column, where the pressure in the upper region of the column is set at a higher level than in the lower region of the column, and the difference in pressure between the upper and lower regions of the column is from 10 to 100 mbar, and the temperature in the lower region of the column is set at a higher level than in the upper region of the column, with the difference in temperature between the upper and lower regions of the column being from 40xc2x0 C. to 210xc2x0 C. The setting up according to the invention of a predetermined concentration profile in the column makes it possible for the formic acid to be kept in the upper part of the column without the acid breaking through into the N-alkyl-Nxe2x80x2-methylalkyleneurea removed as product from the lower part of the column.
It is preferred for the difference in pressure between the upper region of the column, i.e. the top of the column, and the lower region of the column, i.e. the bottom of the column, to be from 20 to 40 mbar and for the difference in temperature to be from 80xc2x0 C. to 150xc2x0 C.
The N-alkyl-Nxe2x80x2-methylalkyleneurea can be removed in gas form from the lower region of the column. The column which is used preferably has a packing with at least about 30 theoretical plates.
The amount of formic acid still present in the reaction mixture after the reaction is up to 35% by weight. xe2x80x9cEssentially formic acid-freexe2x80x9d means for the purpose of this invention a formic acid content in the final product of less than 0.1%. The actual reaction of the appropriate alkyleneureas with formaldehyde can be carried out using monomeric or polymerized formaldehyde, such as paraformaldehyde.
In a preferred embodiment of the process according to the invention, the appropriate alkyleneurea is reacted with the formaldehyde and the formic acid at a temperature of from 70 to 150xc2x0 C. In this connection, the reflux temperature of the reaction mixture under atmospheric pressure, which corresponds to a temperature of about 100 to 110xc2x0 C., is particularly preferred. It is subsequently possible first to remove the mixture, produced in this reaction, of formic acid and water by distillation. Then, in another step, the considerable amount of formic acid still remaining is removed from the required product.
It has proven particularly suitable to carry out the process in two stages, obtaining the N-alkyl-Nxe2x80x2-methylalkyleneurea which is essentially formic acid-free according to the invention in a first step, and carrying out a final distillation to remove near boilers and high boilers in a second step. This increases the space-time yield.
The process is particularly preferably used to prepare N,Nxe2x80x2-dimethylpropylene- and -ethyleneurea.